It is known to use a hydraulic transmission with a variable displacement hydraulic pump and a variable displacement hydraulic motor in apparatus such as wind turbine generators and vehicles. For example, in the case of a wind turbine generator, a variable displacement hydraulic pump may be driven by a drive shaft connected to a rotor which is driven by the wind, and one or more variable displacement hydraulic motors may be connected to one or more electrical generators, and driven by pressurised working fluid from the output of the hydraulic pump. In the case of a vehicle, an internal construction engine or battery may drive the hydraulic pump and a hydraulic motor may drive each wheel, or other actuator.
Suitable variable displacement hydraulic pumps and motors include those which comprise a rotating shaft and a plurality of cylinders of cyclically varying working volume, in which the displacement of working fluid through each cylinder is regulated by electronically controllable valves, on each cycle of cylinder working volume, and in phased relationship to cycles of cylinder working volume, to determine the net throughput of working fluid by the machine. For example, EP 0361927 disclosed a method of controlling the net throughput of working fluid through a multi-cylinder pump by opening and/or closing electronically controllable valves, in phased relationship to cycles of cylinder working volume, to regulate fluid communication between individual cylinders of the pump and a low pressure working fluid line. As a result, individual cylinders are selectable by a valve control module, on each cycle of cylinder working volume, to either displace a predetermined fixed volume of working fluid (an active cycle), or to undergo an inactive cycle (also referred to as an idle cycle) in which there is no net displacement of working fluid, thereby enabling the net throughput of the pump to be matched dynamically to demand. EP 0494236 developed this principle and included electronically controllable poppet valves, which regulate fluid communication between individual cylinders and a high pressure working fluid line, thereby facilitating the provision of a hydraulic motor (which in some embodiments may function as a pump or a motor in alternative operating modes). EP 1537333 introduced the possibility of active cycles in which only part of the maximum displacement of an individual cylinder was selected.
Wind turbine generators, vehicles, or other machines including hydraulic transmissions, may be damaged by resonant oscillations arising from the operation of the machine including resonant oscillations arising from the operation of the hydraulic transmission. For example, EP 2146093 discloses a method and arrangement for damping oscillation in the tower of a wind turbine generator by controlling a power offset signal. U.S. Pat. No. 7,309,930 discloses a vibration damping system and method in which oscillations of the turbine tower are damped by controlling the torque produced by the generator. EP 1719910 discloses a method of actively damping vibrations in a wind turbine tower in which the pitch angle of the wind turbine blades is controlled.
However, it has been found that when employing hydraulic pumps and motors of the type described above, vibrations may arise, resulting from the pulsatile nature of the flow through the hydraulic pump, or motor, which may lead to oscillations if they coincide with a resonant frequency of one or more components. Vibrations may arise which are dependent on the frequency with which active cycles are selected. For example, if ten active cycles are selected per second, spaced equally apart in time, vibrations may arise at 10 Hz. Similarly, problems may also arise from vibrations associated with the frequency of inactive cycles of cylinder working volume. For example, if 90% of cylinders undertake an active cycle and one cylinder per second carries out an inactive cycle, spaced equally apart in time, there may be a vibration of 10 Hz, as a result. Such vibrations can be more damaging, simply because they become relevant when the pump or motor is operating at a high proportion of maximum displacement, and therefore in circumstances where there is a high power throughput, and greater forces are acting.
It is difficult to avoid resonances arising from these vibrations because of the wide range of conditions under which wind turbine generators and other machines may operate, and the complex factors which determine what vibrations may be generated by hydraulic pumps or motors of the above type.